Ultimately for any metric, you'd need to look at the devices you are polling and what metrics are supported for those devices.
1) For some example devices, you can look at the DA Device Management screen for those devices:
Administration->Monitored Items Management->Monitored Devices
Find the device in the tree or use the search tab
Click the Metrics tab to see the metrics supported by the different Metric Families and Vendor Certifications
Note that if there is no Metric Expression, that metric is not collected by the referenced Vendor Certification.
2) Another place to look is the certification portal here:
You can see what metrics are supported for the different devices.
If you don't see the metrics you are looking for that can be submitted as a certification request:
Note that certification requests do take some time.
Other alternatives are self-certification where you use the documentation to certify needed metrics :